This invention concerns a method of growing single crystal metal seed bars, and also a method of casting an item, and particularly but not exclusively casting a turbine blade for a gas turbine engine.
Single crystal casting of metals is used in a number of applications, for instance producing items such as turbine blades for gas turbine engines. For some such articles the crystal orientation in the blades can be quite significant, and hence the use of single crystal casting.
Conventionally a spiral helix grain selector is used to select a single grain. A spiral is also used to eliminate stray grains nucleated at the seed melt-back which could subsequently grow competitively with the primary single crystal. Using a spiral helix involves a number of operations and may provide difficulties in seed to blade alignment in products such as turbine blades. Also when using spiral helixes, spiral cracking can occur during de-waxing operations. Also when using spiral helixes stray grains can nucleate within the spiral because of local front curvature.